Self-sustained unsteady turbulent flow resulting from a IV typed shock-shock interactionis simulated numerically. Full N-S equations implemented with thealgebraic Baldwin-Lomax model are solved by using finite volume method,second-order Harten-TVD spatial scheme and second-order Runge-Kutta method.Regular oscillatoins and a periodic structure of dual eddies areobserved, which do not exist in steady cases. The peak pressure alsooscillates regularly in its value and position, but theirvariation are very small. Time-averaged wall pressure coefficient and Stanton numberdistributions appear in good agreement with steady experimental results.From variationsof three representative lines in the flow field, the inherent unsteady mechanism and influencefactors are then analyzed by studying disturbances propagating in one cycle, and the phases ofthe structure variation. The disturbances propagate through subsonic areas, shocks and shear layers.Because of different structures having different phases, thedisturbances finally result a regular periodicflow field. It shows that the phase difference between two shear layers near the wall, and thelength difference between shear layers abuting against the supersonicjet are two facts that influence the flow field significantly.